Many ophthalmic surgical procedures can be performed by photodisrupting or cutting the targeted ophthalmic tissue with a laser beam of femtosecond pulses. The cuts can be created by scanning the focus spot of the laser beam in a two or three dimensional scan-pattern. Each femtosecond pulse can create a plasma or cavitation bubble at the focus spot of the laser. A layer of these microscopic bubbles can separate the targeted tissue into two parts, creating a macroscopic surgical cut. A volume of these bubbles can cause a volumetric photodisruption of the target tissue.
The minimum pulse energy density capable of creating the plasma in a tissue is called the plasma threshold. The plasma threshold is specific for each ophthalmic tissue. Pulses with energy density below the plasma threshold do not create bubbles. On the other hand, pulses with energy density substantially exceeding the plasma threshold can cause collateral damage and unwanted thermal effects in the target tissue. Therefore, existing ophthalmic surgical systems are typically designed to deliver laser pulses with energy density exceeding the plasma threshold, but only moderately. Such systems can create the desired cuts in the target tissue with limited or minimal thermal effects and collateral damage caused by the excess energy of the laser pulses.
Even if the system was designed to deliver a laser beam with energy density exceeding the plasma threshold along the entire scan-pattern, in real-life situations the energy density of the laser beam can dip below the plasma threshold along a portion of the scan-pattern. In those portions the target tissue will not be photodisrupted or cut, diminishing the efficacy of the ophthalmic surgical procedure. Therefore, it is a high priority challenge to design surgical laser systems that can keep the energy density of the laser beam slightly above the plasma threshold throughout the complete scan-patterns of ophthalmic surgical processes.
Several factors impact the energy density of the laser beam. The energy density is proportional to the peak intensity of the laser beam at the focus spot and inversely proportional to the area of the focus spot. The area of the focus spot is typically of the order of a (2-5 micrometers)2. This focus-spot-area depends on the numerical aperture of the laser beam, on the quality of the optics of the surgical laser system and on the scattering the laser beam experiences while traveling through the ophthalmic tissue after it left the optics, among others. The area of the focus spot cannot be reduced below its diffraction-limited value, set by the wave nature of light. In principle, some of the most efficient optics are capable of delivering their laser beam with a focus spot close to the diffraction limited value throughout the scan-pattern within the surgical volume.
Unfortunately, in reality the laser beams often fall short of being diffraction limited in at least parts of the scan-pattern due to distortions in the laser beam itself, distortions in the beam scanning-focusing optics and distortions in the tissue itself. Therefore, it remains a challenge to design systems and methods that reduce and minimize the beam distortions even in real-life applications so that the laser beam can be delivered with an energy density that exceeds the plasma threshold throughout the entire scan-pattern but only by a small amount during ophthalmic surgeries.